1. Field of the Invention
The present invention relates to a solid oxide fuel cell, more particularly relates to a solid oxide fuel cell having a supported electrolyte film suitable for distributed power sources or cogeneration systems in cities, and fuel cells used in automobiles.
2. Description of Related Art
The solid oxide fuel cell (referred to as SOFC hereinafter) is a fuel cell that employs oxide ion-conductive solid electrolytes as the electrolyte. Its structure is roughly classified into a cylindrical type, a planar type, and a monoblock layer type. Among them, the planar type exhibits high efficiency in power generation due to its relatively low internal resistance as well as high power densities per unit volume deriving from its laminated structure comprised of multiple thin cells.
The planar type SOFC is further divided into a self-supporting electrolyte film type and a supported electrolyte film type. In FIG. 5A is shown a schematic block diagram for an SOFC of self-supporting electrolyte film type. In the figure, an SOFC 10 having a self-supporting electrolyte film has a structure, in which a thin fuel electrode 14 and an air electrode 16 are bonded to both sides of a self-supporting electrolyte film 12 having a thickness of 300 to 500 xcexcm to form an electrolyte-electrode assembly 18, and the electrolyte-electrode assembly 18 is interposed between two gas separators 20, 20.
As the operating temperature of the SOFC 10 having a self-supporting electrolyte film is generally about 1,000xc2x0 C., reactions in a cell proceed rapidly. It also allows internal reforming, and facilitates minimization and efficiency improvement for the cell. In addition, there have been many research reports on the SOFC having a self-supporting electrolyte film and its reliability and durability has been proved.
The SOFC 10 having a self-supporting electrolyte film, however, requires a rather long time period for elevating temperature to its operating range. Expensive heat-resistant materials are needed for the surrounding parts of the SOFC 10, and its gas sealing is also difficult to deal with. In order to avoid these problems, operating temperature of the SOFC 10 should be lowered. But simple lowering of the operating temperature leads to increasing resistance in the electrolyte film 12, which does not induce higher outputs. This is because the conductivity of oxide ion-conductive solid electrolytes generally depends on the temperature, and decreases as the temperature decreases.
On the other hand, an SOFC 30 having a supported electrolyte film, as shown in FIG. 5B, comprises a structure, in which a very thin electrolyte film 32 is supported by a thick fuel electrode 34 (referred to as fuel electrode substrate hereinafter), and a thin air electrode 8 is bonded to the other surface of the electrolyte film 32 forming an electrolyte-electrode assembly 38, and the electrolyte-electrode assembly 38 is interposed between two gas separators 40, 40.
As the resistance of the electrolyte film 32 is proportional to its specific resistance and thickness, the total resistance of the electrolyte film 32 can be lowered by thinning its thickness even if the specific resistance of the electrolyte film 32 is increased. For this reason, the operating temperature for the SOFC 30 having a supported electrolyte film can be lowered to 700 to 850xc2x0 C. without diminishing its outputs. In addition, inexpensive materials such as stainless steel can be used for the surrounding parts resulting in cost down of the SOFC 30. Further compression of temperature-raising time and facile gas sealing may be accomplished so that the utility and durability of the SOFC 30 are improved more.
In the fuel electrode substrate 34 for the SOFC 30 having a supported electrolyte film, a cermet comprising nickel and yttria-stabilized zirconia (referred to as 8YSZ hereinafter), which has a composition of ZrO2 containing 8 mol % Y2O3, is employed. 8YSZ is also commonly used for the electrolyte film 32. (For example, refer to xe2x80x9cSOLID OXIDE FUEL CELL VIxe2x80x9d, S. C. Singhal, M. Dokiya (ed.), p822-p829. )
The fuel electrode substrate works to donate and accept electrons, at the same time supplies fuel gas nearby the electrolyte film, and discharges reacted products out of the system. It is comprised of porous materials having the gas permeability. 8YSZ itself is a material having rather small mechanical strength. Therefore, the conventional SOFC having a supported electrolyte film using Ni-8YSZ as the fuel electrode substrate has been accompanied with a problem that the electric cell easily cracks and thus has a low reliability.
On the other hand, the reliability of an SOFC having a supported electrolyte film using Ni-8YSZ has been improved using a thicker fuel electrode substrate as its mechanical strength is reinforced. But the thicker substrate causes inferior gas permeability resulting in lowered outputs of an SOFC. In the case of a planar SOFC, multiple cells are generally employed in laminated stack. Thus a thicker fuel electrode substrate increases a necessary amount of raw materials, and material costs and thickness of the stack in total, and decreases output density per unit volume.
In order to improve outputs of an SOFC having a supported electrolyte film, it is essential to lower the resistance of the electrolyte film. On the other hand, the oxide ion conductivity of a solid electrolyte is generally determined by its composition. Therefore, in order to lower the resistance of the electrolyte film at low temperatures retaining conventional 8YSZ, the thickness of electrolyte film must be further decreased. However, the electrolyte film is needed of a separator function for isolating reaction gases. A thinner electrolyte film yields difficulties in retaining its gastight quality. So there are certain limits for raising outputs of the SOFC having a supported electrolyte film by thinning the electrolyte film.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide an SOFC having a supported electrolyte film, which is exceedingly reliable, yielding high outputs, and exhibiting high output densities per unit volume.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, a solid oxide fuel cell having a supported electrolyte film comprises an electrolyte film comprised of a first solid electrolyte exhibiting oxide ion conductivity, a fuel electrode substrate which is bonded to a surface of the electrolyte film, and an air electrode which is bonded to the other surface of the electrolyte film forming in total an electrolyte-electrode assembly, wherein the fuel electrode substrate is characterized by comprising a cermet of a first catalyst and a second solid electrolyte which shows oxide ion conductivity and has a bending strength of 500 MPa or more.
The present invention employs a cermet comprised of the first catalyst and the second solid electrolyte possessing a high mechanical strength as the fuel electrode substrate for an SOFC having a supported electrolyte film. Therefore, it effects the fuel cell less likely to crack leading to improved reliability. Also the fuel electrode substrate can be made thinner to improve the output density per unit volume.
In addition, when scandia-stabilized zirconia containing 9 to 12 mol % scandia (Sc2O3) is used for the first solid electrolyte constituting an electrolyte film, the specific resistance of the electrolyte film itself is decreased so that the power output of an SOFC having a supported electrolyte film is further improved.
Furthermore, installation of an intermediate cermet layer comprising the second catalyst and the third solid electrolyte possessing high oxide ion conductivity at low temperatures between the electrolyte film and the fuel electrode substrate causes reduction in the interfacial resistance between the electrolyte film and the fuel electrode substrate, effecting further improvement in the power generation performance of the SOFC.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.